Interference by various kinds of signals causes deterioration in received quality of a signal which a base station apparatus receives. An array antenna has been known as a technology for controlling the interference, and for strongly receiving only a signal arriving from a desired direction. The array antenna comprises a plurality of antenna elements, and is an antenna for arbitrarily setting a directivity by adjusting the amplitudes and phases of signals received through each antenna element. By using the array antenna, only a signal arriving from a desired direction may be strongly received by adjusting a weighting coefficient (hereinafter, called as “weight”) by which the received signal is multiplied, and by regulating the amplitudes and phases of the received signal.
Moreover, there is a RAKE receiving technology as a technology for improving the received quality. By using the RAKE receiving, path diversity gain may be obtained by combining signals arriving through different paths from each other under a multipath environment on a time axis. Generally, even in a base station apparatus provided with the array antenna, the RAKE receiving is often used together.
In this case, the base station apparatus estimates each direction-of-arrival for signals arriving through paths respectively, and receives signals through the array antenna forming directivities in the estimated directions. Hereinafter, a conventional base station apparatus performing the array receiving and the RAKE receiving together will be described.
FIG. 1 is a block diagram showing a configuration of the conventional base station apparatus. In the base station apparatus, signals received through antennas 11-1 to 11-n are input to despreading sections 13-1 to 13-m after predetermined radio processing (such as down conversion, and analog to digital conversion) in RF sections 12-1 to 12-n provided corresponding to each antenna.
The despreading sections 13-1 to 13-m perform despreading processing of signals arriving respectively through a first path to an m-th path. That is, the despreading sections 13-1 to 13-m perform the despreading processing according to receiving timing of the signals arriving through each path. Thereby, signals passed through the first path (hereinafter, called as “the path-1 signals”) which have been received through the antennas 11-1 to 11-n are extracted by the despreading sections 13-1, respectively, and signals passed through the m-th path (hereinafter, called as “the path-m signals”) which have been received through the antennas 11-1 to 11-n are extracted by the despreading sections 13-m, respectively. N number of the path-1 signals are input to a direction-of-arrival estimation section 14-1 and an array receiving section 15-1, and N number of the path-m signals are input to a direction-of-arrival estimation section 14-m and an array receiving section 15-m.
In the direction-of-arrival estimation sections 14-1 to 14-m, directions-of-arrival θ1 to θm of the path-1 to path-m signals are estimated. The estimated directions-of-arrival θ1 to θm are input to the array receiving sections 15-1 to 15-m, respectively.
In the array receiving section 15-1, the path-1 signals received through each antenna are multiplied by a receiving weight generated by using the direction-of-arrival θ1, and, then, the signals subjected to the multiplication are combined. Thereby, the path-1 signals subjected to array combining are output from the array receiving section 15-1. Similarly, the path-m signals subjected to array combining are output from the array receiving section 15-m.
Channel fluctuations h1 to hm are compensated by multiplication of the signals subjected to array combining by complex conjugates h1′* to hm′* of channel estimation values h1′ to hm′, and, thereafter, RAKE combining of the signals is performed in a RAKE combining section 16. The signals subjected to the RAKE combining are demodulated in a demodulation section 17, and, thereby, demodulated data are obtained.
Subsequently, operations for estimating a direction-of-arrival, which are executed in the above conventional base station apparatus, will be described. Here, it is assumed for easy description that received signals arrive through two paths of the path 1 and the path 2, and an array antenna comprises two antenna elements of the antennas 11-1 and 11-2. In a word, description will be made under assumption of m=2 and n=2 in the block diagram shown in FIG. 1. Moreover, it is assumed that each antenna element is arranged straight at intervals of the half wave-length of a carrier wave.
Generally, the path-m signal received through an n-th antenna element may be expressed as the following formula (1).xnm(t)=hmejπ(n−1)sin θm·s(t)  (1)
In the above formula (1), hm represents a channel fluctuation on the path m; θm a direction-of-arrival of the path-m signal; and s(t) a desired signal.
Moreover, ejπ(n−1)sin θm represents a phase rotation between each antenna element. Here, it is assumed for easy description that interference components and noise components included in the received signal may be neglected.
Accordingly, the path-1 signal received through the antenna 11-1 is expressed as the following formula (2); the path-1 signal received through the antenna 11-2 is expressed as the following formula (3); the path-2 signal received through the antenna 11-1 is expressed as the following formula (4); and the path-2 signal received through the antenna 11-2 is expressed as the following formula (5):x11(t)=h1·s(t)  (2)x21(t)=h1ejπ sin θ1·s(t)  (3)x12(t)=h2·s(t)  (4)x22(t)=h2ejπ sin θ2·s(t)  (5)
The direction-of-arrival estimation section 14-1 estimates a direction-of-arrival θ1, using the signal expressed in the above formula (2), and the signal expressed in the above formula (3). Similarly, the direction-of-arrival estimation section 14-2 estimates a direction-of-arrival θ2, using the signal expressed in the above formula (4), and the signal expressed in the above formula (5). Specifically, the direction-of-arrival is estimated according to the following procedure. Here, it is assumed that a beamformer method is used as one example of methods for estimating the direction-of-arrival.
In the beamformer method, a receiving weight vector W(θ) by which the signals received through each antenna are multiplied is assumed to be the following formula (6):W(θ)=[1, e−jπ sin θ, . . . , e−j(n−1)π sin θ]  (6)
Then, directions-of-arrival for signals of each path are estimated by detecting peak positions of angle spectrums PBF(θ), which is represented by the following formula (7) using an auto-correlation matrix RXX of a received signal X.                                                         P                              B                ⁢                                                                   ⁢                F                                      ⁡                          (              θ              )                                =                                                                      W                  H                                ⁡                                  (                  θ                  )                                            ⁢                              R                                  x                  ⁢                                                                           ⁢                  x                                            ⁢                              W                ⁡                                  (                  θ                  )                                                                                                      W                  H                                ⁡                                  (                  θ                  )                                            ⁢                              W                ⁡                                  (                  θ                  )                                                                    ,                              R                          x              ⁢                                                           ⁢              x                                =                      E            ⁡                          [                                                X                  ⁡                                      (                    t                    )                                                  ⁢                                                      X                    H                                    ⁡                                      (                    t                    )                                                              ]                                                          (        7        )            
Here, H represents a complex conjugate transposition, and E an average in the above formula (7).
That is, in the direction-of-arrival estimation section 14-1, the signal expressed by the above formula (2), and the signal expressed by the above formula (3) are multiplied by receiving weight vectors expressed by the above formula (6), respectively, and, thereafter, angle spectrums are obtained by the above formula (7), with changing θ. Then, a peak position on the angle spectrum is estimated as the direction-of-arrival θ1 of the path-1 signal. For example, when an angle spectrum for the path-1 signal is as shown in FIG. 2A, it is estimated that the direction-of-arrival θ1 is a direction of −40 degrees.
Similarly, in the direction-of-arrival estimation section 14-2, the signal expressed by the above formula (4), and the signal expressed by the above formula (5) are multiplied by receiving weight vectors expressed by the above formula (6), respectively, and, thereafter, angle spectrums are obtained by the above formula (7), with changing θ. Then, a peak position on the angle spectrum is estimated as the direction-of-arrival θ2 of the path-2 signal. For example, when the angle spectrum for the path-2 signal is as shown in FIG. 2B, it is estimated that the direction-of-arrival θ2 is a direction of 0 degrees.
However, as angle spectrums are obtained for each path, direction-of-arrival estimation sections for one same operation is required to be provided for each path in the conventional base station apparatus with the above configuration, there has been caused a problem that the apparatus size is increased. When m number of path signals are subjected to RAKE combining, it is required to provide m number of direction-of-arrival estimation sections for one same operation.
Moreover, as operations as expressed in the above formula (6) and the above formula (7) are performed in order to obtain an angle spectrum, there has been a problem in the above conventional base station apparatus that the amount of operations is exponentially increased according to increase in the number of antenna elements and the number of target paths for the RAKE combining.
Moreover, as the above configuration is provided for each communication terminal as shown in FIG. 1, the apparatus size and the amount of operations are further increased, when the number of communication terminals (that is, a number of channels) that the base station apparatus can simultaneously communicate with. The apparatus size and the amount of operations in the base station apparatus have still more increasing tendency along with recent remarkable increase in the number of communication-terminal users.